In the wave guide of a linear accelerator, electrons are accelerated at high energy. The electrons leave the wave guide through an exit window and enter a collimator assembly. The collimator assembly forms an electron field of a given size at a predetermined distance from the exit window. This electron field is applied to the patient under treatment. The end part of the collimator assembly which is next to the patient will be called herein the "electron applicator". In some linear accelerators, (see, for instance, brochure "Mevatron 12", Siemens AG Erlangen, West Germany, No. MT 3/7137), the total length from the exit window to the end of the applicator is chosen to have a definite value, for instance, 1 meter, to define the dose rate (for example, 300 R/min) of electron radiation.
It is desirable to have applicators with different end openings and therefore different electron field sizes for the treatment of malicious tissues of different sizes. It is also desirable to have applicators which do not hurt the patient in case the patient should inadvertantly touch the end of the applicator.
In the field of linear accelerators, the so-called Henschke electron applicator is known. This applicator is essentially an assembly of an outer or support tube and an inner or insert tube. The support tube is stationary and connected to the collimator. An upper portion of the insert tube is slideably mounted inside the support tube. Both tubes are of cylindrical shape.
Electron collimators and electron applicators are known in the field of accelerators. An electron beam collimator is described in U.S. Pat. No. 4,053,808, and an electron applicator is disclosed in U.S. patent application Ser. No. 871,200 now U.S. Pat. No. 4,220,866.